Abstract: From the reaction of Mo_2S_4(μ-dtp)(dtp)_3(H_2O) (I) (dtp = diethyldithiophosphanato) with 2,2'-bipyridine, the product Mo_3S_4(μ-dtp)(η~2-dtp)_2(η~1-dtp)(bpy)(Ⅱ) has been isolated, and characterized by X-ray crystallography. Crystal: (Ⅱ) belongs to triclinic system in space group Pi with crystallographic parameters as following., a=1.3638(3), b=1.3983(3), c=1.4699(2) nm, α=95.57°, β=91.87°, γ=117.40°; V=2.467(2) nm, Z=2, Dc=1.767 g.cm~(-3), final R=0.069, R_w= 0.076. The results of the structure determination show that the coordinated H_2O ligand and one of sulphur atoms of dtp on the Mo(3) atom of cluser(Ⅰ) have been substituted by 2,2′-bipyridine ligand. It has been pointed out that the ligand substitution reaction reported here is closely correlated with the existence of loosely-coordinated site in cluster (Ⅰ). In addition, the other types of substitution reactions of ligands are also discussed.